The present invention relates generally to a radar system which uses Global Positioning System (GPS) satellites as radiation sources and uses GPS receivers to measure the range from a target to an observation point.
Passive ranging data is critically needed information in military operations. Several decades of study on this technology provides very little new information. One conventional approach is to measure the angle of arrival of a target when the target is intentionally emitting electromagmetic energy. The angle of arrival information is measured at several positions along the flight path and a triangulation method is used to determine the range to the target. This approach requires very accurate angle of arrival measurements and a relatively long time to obtain range accuracy.
Measuring the distance between a certain target and an observation point using GPS signals is faster, convenient and more accurate than conventional approaches. The well-known Navstar GPS includes 24 spacecraft in orbits inclined at 55 degrees to the Equator. The inclined orbits provide worldwide coverage, including the North and South poles. The GPS system allows a user anywhere on Earth to receive the transmissions of at least four satellites at once. Triangulation mathematical calculations with these satellites provide a very accurate reading of position and velocity in three dimensions. Control stations around the world keep GPS satellites precisely calibrated and their orbits aligned. Each GPS satellite contains an atomic clock and transmits a continuous time signal and other information to receivers on Earth. The receiver must acquire and track these signals, decode the data, and then make range and velocity calculations.
The approach of the invention uses GPS satellites as radiation sources and GPS receivers to form a passive radar system. GPS signals have two unique characteristics which are desirable in passive range measuring. First, the signals are always available from four or more different satellites. Second, the GPS continuous time coarse/acquisition (C/A) signal has a period of 1 millisecond, thus, theoretically it is possible to measure the distance every millisecond. Because of these two characteristics, one can use GPS signals to measure distance at any location, any time and perform updates frequently. U.S. Pat. No. 5,187,485, Tsui, et al., incorporated herein by reference, discloses and claims a method to passively measure the range to a target using GPS satellites. The present invention provides an improved method for determining the distance from a target to an observation point using GPS satellites.